The present invention relates to a ferroelectric liquid crystal screen for the display of information such as images or characters having opacified electrodes in the non-switchable area of the screen, to a process for obtaining spacers for said screen and to a process for treating the screen.
1. Field of the Invention
The invention may be practiced with tilted chiral smectic C,I,F,G or H liquid crystals and in particular with chiral smectic C phase liquid crystals.
2. Description of the Related Art
EP-A-0 032 362 describes a display means, whose electrooptical display material is a chiral smectic C phase liquid crystal. This display means, diagrammatically shown in longitudinal sectional form in FIG. 1, has a first linear polarizer 2 and a second linear polarizer 4, which cross one another and between which is inserted a light display cell 6. A light source 8 located below polarizer 4 makes it possible to illuminate cell 6.
This display cell operating in transmission is formed by two electrically insulating, transparent walls or plates 10,12, which are generally of glass. These parallel substrates are joined by their edges by a bonded joint 14 serving as a sealing joint.
Walls 10 and 11 are respectively covered by an electrode 16 and a counterelectrode 18 having a shape appropriate for the display and made from a transparent conductive material. The electrode and counterelectrode can in each case be formed by parallel conductive strips, the strips of the electrode, which will be called the column electrode and the strips of the counterelectrode which will be called the row electrodes perpendicularly cross one another.
The electrode and the counterelectrode make it possible to apply to the terminals of a chiral smectic C phase liquid crystal film 20 contained in cell 6, a continuous electric field E the direction or value of which can be modified. To this end, electrode 16 and counterelectrode 18 are connected, via an inverter 22, to a continuous electric power supply 24.
FIG. 2 shows on the molecular scale the structure of a smectic C phase liquid crystal film, when the latter is contained in display cell 6. With a view to simplifying FIG. 2, all that is shown is the cell walls 10 and 12. The lower wall 12 e.g. constitutes a reference plane containing the two axes X and Y of an orthogonal reference system XYZ.
The smectic C liquid crystal film is formed by elongated molecules 26 having a longitudinal axis 28 and arranged in layers 30. Each of these molecules has a permanent dipole moment p perpendicular to their longitudinal axis 28.
In the ideal case shown in FIG. 2, the smectic layers 30 are all parallel to one another and oriented perpendicular to the cell walls 10 and 12.
When an electric field E is applied to such a liquid crystal, a high coupling is obtained between the molecular orientation (longitudinal axis 28 of the molecules) and said electric field E due to the presence of the permanent dipole. This coupling is of the polar type, because the electric dipole is preferably oriented in a direction parallel to the electric field. The polarity change of the electric field consequently makes it possible to change the orientation of the electric dipole and therefore the orientation of the molecules 26.
FIG. 2 shows in continuous line form the molecules 26 of the liquid crystal in accordance with a first orientation A.sub.1 (state 1) forming an angle -.theta. with respect to the direction X, the dipole moments p being oriented perpendicular to the cell walls 10,12 and in the direction of the electric field E from wall 10 to wall 12. The polarity change of the electric field permits the tilting the dipole moments p in the opposite direction (from wall 12 to wall 10) leading to a pivoting of the molecules about axis Z by an angle of 2.theta.. The second orientation A.sub.2 of the molecules (state 2) is symbolized in dotted line form and forms an angle +.theta. with respect to direction X.
The molecules pass from the first to the second orientation and vice versa describing a cone angle at the apex 2.theta. characteristic of the material (typically .theta.=22.5.degree.).
FIG. 2 also shows the respective polarization directions P and P' of the linear polarizers 2 and 4.
When these two polarizers are crossed and when in state 1 the molecules 26 of the liquid crystal are parallel to the polarization direction P' of the polarizer 4, the optical state 1 of the liquid crystal corresponds to the absorption of the light from source 8 and optical state 2 to the transmission of said same light.
The chiral smectic C phase liquid crystals when appropriately oriented (FIG. 2) can therefore be used as display materials. Apart from their bistability, they can have interesting properties, such as a fast response or switching time of approximately one microsecond for low voltages applied to the electrodes (a few volts) and a wide electrooptical response.
For the apparatus shown in FIG. 1 to operate correctly, the liquid crystal thickness must be extremely small, e.g. approximately two micrometers. The spacing of walls 10 and 12 leading to such a thickness is generally obtained by means of spacers constituted by calibrated plastic balls. These balls when used as spacers are arranged in a random manner between walls 10 and 12.
FIG. 3 shows very diagrammatically and in plan view a liquid crystal display screen comprising transparent, parallel row electrodes 32 and transparent, parallel column electrodes 34, which are perpendicular to the row electrodes.
One of the most important parameters of this screen is the contrast obtained between the displayed black state N and the white state B. This contrast is defined by the ratio of the intensity transmitted in the white state IB to the intensity transmitted in the black state IN. In order to obtain a high contrast, it is necessary for the intensity of the black state, e.g. corresponding to state 1 of the apparatus described relative to FIGS. 1 and 2, and the white state then corresponding to state 2, to be as low as possible, so as to have a large IB/IN ratio.
When the screen shown in FIG. 3 uses an e.g. chiral smectic C phase, bistable ferroelectric liquid crystal, the non-switchable area 36 of the screen contains densities substantially equal to states 1 and states 2. The switchable area 38 in FIG. 3 corresponds to all the "overlaps" of electrodes 32 and 34 (in plan view) and that the non-switchable area (or non-addressable area) corresponds to the remainder of the screen. Thus, to obtain a good bistability, the surface treatments permitting the orientation of the liquid crystal are such that the two states are equiprobable in the non-switchable area.
The non-switchable area 36 therefore appears grey when an appropriate voltage is established between electrodes 32 and 34 and the linear polarizers are appropriately positioned on either side of the assembly or cell comprising electrodes 32, 34 (respectively placed on the generally glass, electrically insulating plates) and the liquid crystal layer.
The fact that the non-switchable area appears grey is highly prejudicial to the contrast, even if said electrooptical effect used makes it possible to obtain an excellent black state at the switchable area 38.
The dimensions of the non-switchable area cannot be significantly reduced, because for large complex screens, the efficiency of the etching operations necessary for their production impose a limit size to the non-switchable area.
This problem relating to the grey appearance of the non-switchable area is also encountered with screens using other liquid crystals. The problem in question is then solved by placing between the row electrodes and the column electrodes an opaque screen. The latter is generally made from a coloured. electrically insulating material, whose thickness is necessarily a few micrometers, e.g. 1 to 2 micrometers, in order that the insulating material is sufficiently absorbent. Such a thickness is incompatible with screens using a ferroelectric liquid crystal. Thus, the thickness of the liquid crystal layer does not permit the intersection of coloured insulating material layers arranged in gaps separating the row electrodes and the layers of said same material located in the gaps separating the column electrodes.
It would indeed be conceivable to cover the row and column electrodes with an appropriately thick electrical insulant, which would be transparent in the switchable area and opaque in the non-switchable area in order to solve this problem. However, such a technique would be very unfavourable from the screen operation standpoint, because a considerable part of the electrical energy necessary for the switching of the liquid crystal would be lost in the insulant, whose thickness would be comparable to the "active" thickness of the liquid crystal.
Another problem resulting from the use of a ferroelectric liquid crystal, e.g. a chiral smectic C phase liquid crystal, particularly in connection with the production of a large screen, is due to the presence of characteristic alignment errors that are liable to occur with such a liquid crystal. These errors are known under the name "zigzag" and are in particular referred to in the article by M. A. Handschy et al, published in Ferroelectrics, 1984, Vol. 59, pp. 69 to 116.
These defects or errors, which are in the form of lines, reduce the contrast. Moreover, their distribution can be inhomogeneous over the screen surface, which leads to an inhomogeneous appearance of the picture displayed by the screen, which is unfavourable to obtaining a good quality screen.
The density of these defects is dependent on the liquid crystal used and the surface treatments carried out on the plates between which said liquid crystal is located. One of the possible surface treatments consists of placing on each of said plates a layer of an appropriate material and to rub the said material layers either parallel to the row electrodes or parallel to the column electrodes.
It is very difficult to place the liquid crystal between surfaces of several hundreds of cm.sup.2 without zigzag faults occuring. They occur perpendicular to the rubbing direction of the walls and tend to attach themselves to the balls used as spacers and which stop these defects. However, these balls are distributed in a random manner between the screen plates and in particular in the switchable area thereof, which leads to a poor contrast in a durable manner for such a known ferroelectric liquid crystal screen.